Tonight: The Rise of the 2013 “Mini-Moon”

The final Full Moon of 2013 occurs tonight, and along with it comes something special: the most distant and visually smallest Full Moon of 2013.

Why doesn’t the annual “mini-moon” receive the same fanfare and hype that the yearly perigee – or do you say Proxigean to be uber-obscure – “supermoon” does? The smallest Full Moon of the year does appear to have a public relations problem in this regard. But as you’ll see, the circumstances for this week’s Full Moon are no less fascinating.

The exact timing of tonight’s Full Moon occurs at 4:28 AM EST/9:28 Universal Time (UT) on Tuesday, December 17th. This occurs just two days and 14 hours prior to the Moon reaching apogee on December 19th at 6:50PM EST/23:50 UT at 406,267 kilometres distant. This is one of the three most distant apogees of 2013, and the closest to Full for the year. It’s also with 500 kilometres of the most distant apogee than can occur, as the Moon’s apogee can vary between ~404,000 and 406,700 kilometres distant.

Tonight’s Full Moon will have an apparent angular diameter of around 29.8’ arc minutes, just a shade lower than the usual value quoted of around half a degree or 30’. The visual size of the Moon as seen from the Earth varies about 12% from 34.1’ to 29.3’. Also, the Moon is also about half an Earth radius more distant when it’s on the local horizon versus at the zenith overhead!

This is also the closest Full Moon to the December solstice, which occurs four days later on Saturday, December 21st at 12:11 PM EST/17:11 UT. This marks the start of astronomical summer in the southern hemisphere and the beginning of the winter season in the north. Think of tonight’s Full Moon as a sort of “placeholder,” marking the point at which the Sun will occupy during the June solstice on the Gemini-Taurus border.

This all means that tonight’s Full Moon rides high for northern hemisphere residents towards local midnight. But the “Long Night’s Moon” of 2013 is rather lackluster in terms of declination. While it’s the northernmost Full Moon of 2013 at a declination of +18.7 degrees, it’s a far cry from the maximum declination of +28.72 degrees (the angle of the ecliptic plus the tilt of the Moon’s orbit) that it can achieve. This only occurs every 18.6 years and last occurred in 2006 and will happen again around 2025. We’re currently headed towards a shallow minimum for the Moon’s orbit in 2015. Ancient European and Native American cultures both knew of this cycle of high-flying moons.

Not weird enough? The next “most distant Full Moon of the Year” happens only one lunation later on January 16th… within just 2 hours of apogee! Perhaps January’s Full Moon is due notoriety as a “Super-Mini Moon?” Such a pairing of “mini-moons” last occurred on 2004-2005 and will next occur on 2021-2022.

The footprint for the lunar occultation of M67. (Created by the author using Occult 4.1)

The Moon also visits some other celestial sights this week. After passing five degrees north of Jupiter on December 19th, the Moon heads towards an occultation of the open cluster M67 in the constellation Cancer on December 21st for northern North America. Though the Moon will be waning gibbous, it might just be possible to note the reappearance of the cluster on the Moon’s dark limb. Other occultations for the remainder of December by the Moon include an occultation of Spica on December 27th for northern Asia, Saturn on December 29th for Antarctica, and +3.6th magnitude star Lambda Geminorum for Canada on December 18th.

The passing of the Full Moon also means it will be entering into the morning sky, which also means bad news for viewers of the Ursid meteor shower which peaks on December 22nd and hunters of Comet C/2013 R1 Lovejoy, currently shining at +5th magnitude in the constellation Hercules low in the dawn.

The keen-eyed may notice the Moon also transits through the northern end of the non-zodiacal constellation of Orion on Tuesday, December 17th. Did you know that the Moon can actually stray far enough away from the ecliptic to cross through 18 constellations? The Six non-zodiacal constellations it can transit are: Orion, Ophiuchus, Corvus, Sextans, Auriga and Cetus.

China’s Chang’E-3 spacecraft landed on the Moon just outside of the Bay of Rainbows (Sinus Iridum) near Montes Recti in the northern section of the Mare Imbrium on Saturday, December 14th. The landing site is visible now on the lunar nearside, and can be seen with that new Christmas telescope you’ve been itching to try out. Look for the Sinus Iridum as a wide crescent scarp, a sort of “notch” in the top of Mare Imbrium:

Finding the landing site of Chang’e-3. Photos and graphics by author.

China’s Yutu or “Jade Rabbit” rover has been beaming back some splendid images of the lunar surface!

So don’t let the cold temperatures deter you from exploring the lunar surface, and the strange but fascinating motions of our nearest natural celestial neighbor. Dress warm and be sure this Christmas season to raise a glass of ye ole Nog to the Solstice/Yule Moon.